1. Field of the Invention
The present invention relates to an armature in which the conductors of the lead-out section of an armature coil and the segments of a commutator are welded by an ultrasonic welding method, and a method for manufacturing same.
2. Description of Related Art
FIG. 6 is a sectional view showing an essential portion a conventional armature; FIG. 7 is a cross-sectional view taken along line VII--VII of FIG. 6; and FIG. 8 is a cross-sectional view taken along line VIII--VIII of FIG. 6.
An armature 1 is equipped with a shaft 2, an armature core 3 secured to the shaft 2, an armature coil 4 composed of conductors 4b wound in slots 3a formed in the armature core 3, a commutator 5 secured to the shaft 2, and a molded portion 6 which is made of an epoxy resin and which covers the armature core 3 and a lead-out section 4a of the armature coil 4.
The commutator 5 is equipped with a bushing 7 secured to the shaft 2, a plurality of segments 8 which are disposed at equal intervals dircumferentially around the shaft 2 and to which the conductors 4b of the lead-out section 4a are connected, and a molded member 9 which combines the segments 8 and the bushing 7 into one piece.
In the armature 1 configured as described above, the conductors 4b of the lead-out section 4a and the segments 8 are connected by ultrasonic welding. Then, the armature core 3, the armature coil 4, and the commutator 5 are heated to approximately 190 degrees Celsius and powder epoxy resin is electrostatically attached to the outer peripheries of the armature core 3 and the armature coil 4. The epoxy resin is melted by the retained heat of the armature core 3, the armature coil 4, and the commutator 5, and then is hardened by natural radiation cooling. Subsequently, a molded portion 10 is formed to cover the outer peripheral portion of the armature core 3, the lead-out section 4a, and a part of the outer peripheral portion of the commutator 5 as shown in FIG. 9. Afterwards, a portion of the molded portion 10 and an end of the lead-out section 4a are turned and an outer peripheral surface 5a of the commutator 5 that provides a surface with which a brush (not shown) comes into sliding contact, is subjected to finishing, thus completing the armature 1 shown in FIG. 6.
The lead-out section 4a and the segments 8 are welded by ultrasonic welding which makes use of the frictional heat generated from ultrasonic vibration. The ultrasonic welding method is based on solid phase welding behavior and it is essentially different from arc welding or solder welding. The connecting strength obtained by ultrasonic welding is lower than that obtained by arc welding or solder welding, and it is also lower than the strength of the connection between the conductors and the segments that is achieved by heat caulking.
Despite the lower connecting strength, the ultrasonic welding method is used for the following reasons:
(1) There is a danger that connecting the conductors 4b to the segments 8 of the commutator 5 by arc welding or solder welding will cause short-circuiting between adjacent segments, making the welding work difficult. PA1 (2) If a method is employed in which a commutator has a flange with grooves at an end thereof and the conductors are thermally caulked in the grooves to connect the conductors of the armature coil with the segments of the commutator, then the formation of the flange accordingly adds to the amount of copper, the material used for the segments, and it is difficult to machine the grooves when manufacturing the commutator, leading to higher manufacturing cost.
In the conventional armature 1, the epoxy resin used for the molded portion 6 exhibits high viscosity when it is melted, presenting a problem in that insufficient entry of the epoxy resin into the slots 3a of the armature core 3 results in inadequate insulation between the armature core 3 and the armature coil 4 at the slots 3a.
When varnish is used for the molded portion 6 in place of the epoxy resin 6, the lower viscosity of varnish makes it easier for varnish to enter the slots 3a, so that insulation between the armature core 3 and the armature coil 4 is secured. There has been a problem, however, in that the amount of varnish applied to the outer peripheral surface of the armature core 3 and the outer peripheral surface of the lead-out section 4a is small. Accordingly the amount of varnish applied in the vicinity of the welded portion A weakly welded by ultrasonic welding between the conductors 4b of the lead-out section 4a and the segments 8 has been small. This has inevitably resulted in low joining strength between the lead-out section 4a and the commutator 5. Hence, there has been a danger that, when machining or lathe turning a portion of the molded portion 10 and the end of the lead-out section 4a, the cutting force may cause the conductors 4b of the lead-out section 4a to come off the segments 8 at the welded portion A.
Although it is possible to increase the amount of varnish applied to the area in the vicinity of the welded portion A to enhance the joining strength between the lead-out section 4a and the commutator 5, this would require extra time to heat the varnish to harden it and increase the time required to manufacture the armature.